1. The Field of the Invention
The present invention relates to novel liquid crystalline compounds and polymers and methods employing such compounds and polymers as stationary phases in the gas, liquid, and supercritical fluid chromatographic separations and analyses of various substances such as polycyclic aromatic hydrocarbons.
2. The Prior Art
Liquid crystalline compounds and polymers are known for their utility as stationary phases in chromatography. Typically, in such liquid crystalline polymers, mesomorphic side chains of various chemical composition are attached to a polymer backbone. The liquid crystalline polymers are actually crystalline solids at low temperatures, but become "liquid" at higher temperatures, depending upon the specific mesomorphic side chains and polymer backbone employed.
Upon heating the solid crystalline compound, a smectic phase may be reached wherein the compound assumes a more liquid nature. Although in the liquid smectic phase the molecules of the compound are relatively ordered, this order is not as high as that in the crystalline phase. By increasing the temperature, the compound may then enter a nematic phase which is still liquid, but less ordered than the smectic phase. Finally, upon further increase in temperature, the compound may enter an isotropic phase wherein the compound is completely disordered with individual molecules positioned at complete random.
It should be noted that some liquid crystalline compounds and polymers have only a smectic phase or a nematic phase. Compounds that have neither a smectic phase or a nematic phase, however, are not liquid crystals.
Because of their respective degrees of high order and high disorder, the crystalline phase and isotropic phase of liquid crystalline polymers generally do not have utility in chromatographic separation. Thus, those skilled in the art have sought to develop liquid crystalline compounds and polymers having either smectic phases or nematic phases which are suitable for chromatography applications.
In capillary chromatography, achieving maximum separation of components, which is extremely important for successful identification of those components, and achieving maximum resolution of the chromatographic peaks depends on at least two important factors: (1) selectivity and (2) efficiency. Simply stated, selectivity has reference to the distance between the peaks in the resultant chromatogram, whereas efficiency relates to minimizing the width of the peaks. Thus, both selectivity and efficiency affect the resultant resolution and separation of the peaks in the resultant chromatogram. Unfortunately, in the prior art, it has been difficult to maximize both selectivity and efficiency using liquid crystalline compounds and polymers as stationary phases.
In a typical chromatography apparatus, it is desirable to have a stationary phase which is operable over a temperature range from about ambient temperature to about 300.degree.-350.degree. C. Such a temperature range allows gas-liquid chromatography programming to start at ambient temperature and go to higher temperatures as needed to separate the particular chemical components involved. Since the materials from which the gas-liquid chromatographic column and oven are manufactured dictate the limits of this temperature range, it is extremely important that the stationary phase used inside the column also have a versatile temperature range of operation.
The smectic phases of prior art liquid crystalline polymers typically cover much narrower temperature ranges than nematic phases. Thus, the prior art has particularly emphasized the use of liquid crystalline polymers having workable nematic phases. Further in this regard, it should be noted that nematic phases of liquid crystalline polymers usually exhibit better efficiency than smectic phases. However, smectic phases typically exhibit much better selectivity than nematic phases.
Thus, the choice faced in the prior art has been one of selectivity versus efficiency, both of which are needed for good separation results. As mentioned above, the prior art has focused primarily around nematic phase liquid crystalline polymers. In using the nematic phase, the requisite efficiency and temperature range can be obtained at the sacrifice of selectivity. On the other hand, the poor efficiency and narrow temperature ranges of the smectic phases have virtually prohibited usage of the prior art smectic phases in chromatography applications.
Thus, it will be recognized, that what is needed in the art are liquid crystalline compounds and polymers which have good selectivity, good efficiency, and can be used over a wide temperature range so as to provide practical and useful stationary phases in chromatography. Such liquid crystalline compounds and polymers are disclosed and claimed herein.
The prior art has also found that liquid-crystalline stationary phases provide improved capillary gas-liquid chromatography separations of some isomeric species, including some polycyclic aromatic hydrocarbons (PAH), that are difficult or impossible to resolve by other traditional chromatographic techniques. Polycyclic aromatic hydrocarbons are widespread environmental pollutants produced, for example, by the incomplete combustion of organic matter. Members of or derivatives of the PAH class of compounds represent the largest class of proven chemical carcinogens.
The carcinogenic properties of PAH compounds are frequently isomer specific. Thus, it would be highly useful in analyzing the levels of these pollutants to be able to separate PAH into separate isomeric components, and attempts have been made in the prior art to use capillary gas-liquid chromatography to separate such structurally related isomers.
Unfortunately, the liquid crystalline polymers of the prior art have not provided completely satisfactory results in separating PAH. Hence, it would be another advancement in the art to provide liquid crystalline compounds and polymers which could be used as stationary phases in chromatography to satisfactorily separate PAH into separate isomeric components. Such liquid crystalline compounds and polymers are also disclosed and claimed herein.